Exhaust Flange

There are many parts in the car exhaust system that need to be joined together to form a continuous pipe from the engine to the muffler. An exhaust flange is essential for creating a strong joint between two parts of the system, and it also allow the parts to be changed without cutting the exhaust pipe. Single flanges are the most common type, but there are also dual flanges for dual exhaust pipes, and manifold flanges with many holes for the header pipes. Flanges are normally sold in kits that contain several flanges, bolts and gaskets.

The exhaust system begins at the engine where the gases are released into the exhaust manifold. The small pipes of the manifold converge into one or two larger exhaust pipes. Some cars have a turbo after the manifold, which uses the exhaust gases to drive a turbine that injects cold air into the engine. The gases then pass through the catalytic converter, which changes them into less polluting gases, and exit through the muffler. An exhaust flange is attached to each end of the connecting parts so that they can be bolted together instead of welded.

The two-bolt exhaust flange is shaped like a diamond, with a center hole for the pipe and two bolt holes next to it. The three-bolt flange is triangular, with a center hole surrounded by three bolt holes. The basic flange is welded onto the the end of the exhaust pipe, but another type called the ball flange does not require a weld or a gasket. One end of the pipe is curved outwards and the other end curved inwards, so that the two ends of the ball joint press against each other. Flanges are normally made from forged steel or aluminum, with their contact surface machined flat.

When part of the exhaust pipe strikes something on the road, the exhaust flange is subjected to immense stress as the pipe bends. A cracked flange allows exhaust gases to escape and enter the cabin. It is important to repair the flange as soon as possible to prevent the occupants of the car from being poisoned by carbon monoxide. A crack can also affect engine performance in cars fitted with electronic fuel injection. If the crack is before the oxygen sensor, the sensor will register a higher oxygen level. The computer will mistake this for lean running and cause more fuel to be used than is actually required.

If the crack in the flange is small enough, it can sometimes be welded over without removing any part of the exhaust system. However, most cracks require the old flange be removed and a new flange welded in its place. The bolts holding the flanges together are often seized to their nuts because the threads have corroded. An anti-seizing compound can be effective against light corrosion but more severe cases may require that the bolts be cut off with a cold chisel or angle grinder. Once the part has been released, the flange and the welds can be removed with a grinder and hammer.

With the end of the pipe cleaned of welds, a new exhaust flange is placed over the end and welded in place. The part is then installed with a gasket between the flanges. The gaskets are normally made of thin copper sheet that does not compress, so the flanges need to be perfectly flat where they join. While an old gasket can be used if it shows no sign of damage, gaskets are cheap enough that a new one should be used when possible. Springs are sometimes placed around the bolts to give the joint more flexibility and to allow for the effects of thermal expansion.

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